That advice only applies to people maxing out their TSP, and (IMO) even for them it's iffy.
OP-- mathematically the Roth and regular TSP work out the same if your tax bracket stay the same and if you invest the same total amount. If right now you are putting $8000 (pre-tax) into your TSP, that is the same as putting $6000 into the Roth and paying tax of $2000 (assuming that you are in a 25% total marginal tax bracket). In one case you pay the tax now, and have a smaller amount to grow but you get to keep it all, in the other case you essentially have the $2000 in your account but you will need it (and its earnings) to pay tax when you retire and start withdrawals.
The two differences are: 1) if tax rates change for you in the future vs now (obviously you want to pay the taxes whenever they are lower) and 2) the Roth effectively lets you earmark more $ for retirement because you get to save $17.5k and pay tax out of other funds, whereas for your TSP you get to save $17.5k and you'll have to pay taxes out of that money eventually.
The second difference doesn't matter to you if you are not maxing out your contributions, so the question is whether you think you will be in a higher tax bracket in retirement (if it's the same tax bracket it doesn't matter what you choose, if you end up in a lower tax bracket you want to stay in traditional TSP).